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List:       linuxppc-embedded
Subject:    Re: [PATCH] powerpc: Fix definition of SIAR and SDAR registers
From:       Michael Ellerman <mpe () ellerman ! id ! au>
Date:       2016-05-30 23:27:22
Message-ID: 1464650842.16938.7.camel () ellerman ! id ! au
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On Mon, 2016-05-30 at 10:04 +0200, Thomas Huth wrote:
> On 13.05.2016 05:35, Paul Mackerras wrote:
> > On Thu, May 12, 2016 at 01:26:44PM +0200, Thomas Huth wrote:
> > > The SIAR and SDAR registers are available twice, one time as SPRs
> > > 780 / 781 (unprivileged, but read-only), and one time as the SPRs
> > > 796 / 797 (privileged, but read and write). The Linux kernel code
> > > currently uses the unprivileged  SPRs - while this is OK for reading,
> > > writing to that register of course does not work.
> > > Since the KVM code tries to write to this register, too (see the mtspr
> > > in book3s_hv_rmhandlers.S), the contents of this register sometimes get
> > > lost for the guests, e.g. during migration of a VM.
> > > To fix this issue, simply switch to the privileged SPR numbers instead.
> > > 
> > > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > 
> > Acked-by: Paul Mackerras <paulus@ozlabs.org>
> 
> *ping*
> 
> Michael, could you please pick this patch up? I think it should rather
> go through the generic powerpc tree instead of kvm-ppc, since it also
> affects other parts than just KVM...

Yeah that's actually why I hesitated to merge it, because I want to know what
the broader implications are ...

I have also gone back and confirmed that the 796/797 numbers exist and are
correct on all CPUs we support, which involved a lot of digging through PDFs.

cheers

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